1. Field of the Invention
Embodiments of the present disclosure are directed to non-volatile memory technology.
2. Description of the Related Art
Semiconductor memory devices have become more popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. Electrical Erasable Programmable Read Only Memory (EEPROM), including flash EEPROM, and Electronically Programmable Read Only Memory (EPROM) are among the most popular non-volatile semiconductor memories.
One example of a flash memory system uses the NAND structure, which includes arranging multiple transistors in series between two select gates. The transistors in series and the select gates are referred to as a NAND string. FIG. 1 is a top view showing one NAND string. FIG. 2 is an equivalent circuit thereof. The NAND string depicted in FIGS. 1 and 2 includes four transistors 10, 12, 14 and 16 in series between a first select gate 12 and a second select gate 22. Select gate 12 connects the NAND string to bit line terminal 26. Select gate 22 connects the NAND string to source line terminal 28. Select gate 12 is controlled by applying appropriate voltages to control gate 20CG via selection line SGD. Select gate 22 is controlled by applying the appropriate voltages to control gate 22CG via selection line SGS. Each of the transistors 10, 12, 14 and 16 includes a control gate and a floating gate, forming the gate elements of a memory cell. For example, transistor 10 includes control gate 10CG and floating gate 10FG. Transistor 12 includes control gate 12CG and a floating gate 12FG. Transistor 14 includes control gate 14CG and floating gate 14FG. Transistor 16 includes a control gate 16CG and a floating gate 16FG. Control gate 10CG is connected to word line WL3, control gate 12CG is connected to word line WL2, control gate 14CG is connected to word line WL1, and control gate 16CG is connected to word line WL0.
Note that although FIGS. 1 and 2 show four memory cells in the NAND string, the use of four transistors is only provided as an example. A NAND string can have less than four memory cells or more than four memory cells. For example, some NAND strings will include eight memory cells, 16 memory cells, 32 memory cells, etc. The discussion herein is not limited to any particular number of memory cells in a NAND string. A typical architecture for a flash memory system using a NAND structure will include several NAND strings. Relevant examples of NAND-type flash memories and their operation are provided in the following U.S. Patents/Patent Applications, all of which are incorporated herein by reference in their entirety: U.S. Pat. Nos. 5,570,315; 5,774,397; 6,046,935; 5,386,422; 6,456,528; and U.S. patent application Ser. No. 09/893,277 (Publication No. US2003/0002348). Other types of non-volatile memory in addition to NAND flash memory can also be used in accordance with embodiments.
When programming an EEPROM or flash memory device, typically a program voltage is applied to the control gate and the bit line is grounded. Electrons from the channel are injected into the floating gate. When electrons accumulate in the floating gate, the floating gate becomes negatively charged and the threshold voltage of the memory cell is raised so that the memory cell is in a programmed state. The floating gate charge and threshold voltage of the cell can be indicative of a particular state corresponding to stored data. More information about programming can be found in U.S. patent application Ser. No. 10/379,608, titled “Self Boosting Technique,” filed on Mar. 5, 2003; and in U.S. patent application Ser. No. 10/629,068, titled “Detecting Over Programmed Memory,” filed on Jul. 29, 2003, both applications are incorporated herein by reference in their entirety.
Shifts in the apparent charge stored on a floating gate can occur because of coupling of an electric field based on the charge stored in neighboring floating gates. This floating gate to floating gate coupling phenomena is described in U.S. Pat. No. 5,867,429, which is incorporated herein by reference in its entirety. The floating gate to floating gate coupling phenomena occurs most pronouncedly, although not exclusively, between sets of adjacent memory cells that have been programmed at different times. For example, a first memory cell can be programmed to add a level of charge to its floating gate that corresponds to a set of data. Subsequently, one or more adjacent memory cells are programmed to add a level of charge to their floating gates that correspond to a set of data. After one or more of the adjacent memory cells are programmed, the charge level read from the first memory cell may appear to be different than when it was programmed because of the effect of the charge on the adjacent memory cell(s) being coupled to the first memory cell. The coupling from adjacent memory cells can shift the apparent charge level being read from a selected memory cell by a sufficient amount to lead to an erroneous reading of the stored data.
As memory cells continue to shrink in size, the natural programmed and erase distributions of threshold voltages are expected to increase due to short channel effects, greater oxide thickness/coupling ratio variations and more channel dopant fluctuations, reducing the available separation between adjacent states. This effect is much more significant for multi-state memories than binary memories using only two states. The reduction of the space between word lines and between bit lines will also increase the coupling between adjacent floating gates. The effect of the floating gate to floating gate coupling is of greater concern for multi-state devices because the allowed threshold voltage ranges and the forbidden ranges (range between two distinct threshold voltage ranges that represent distinct memory states) are narrower than in binary devices. Therefore, floating gate to floating gate coupling can result in memory cells being shifted from an allowed threshold voltage range to a forbidden range.